Datasheet

LT3032 Series
17
3032ff
For more information www.linear.com/LT3032
APPLICATIONS INFORMATION
25°C during normal operation where the BYPN pin oper-
ates at approximately –60mV. DC leakages on the order
of
1µA into or out of these pins can throw off the internal
reference by 20% or more.
Output Capacitance and Transient Response
The LT3032 requires output capacitors for stability. It
is designed to be stable with most low ESR capacitors
(typically ceramic, tantalum or low ESR electrolytic). A
minimum output capacitor of 2.2μF with an ESR of 3Ω
or less is recommended to prevent oscillations on each
output. The LT3032 is a micropower device and output
transient response is a function of output capacitance.
Larger values of output capacitance decrease peak devia
-
tions and provide improved transient response for larger
load current changes. Additional capacitors, used to de-
couple individual
components powered by the LT3032,
increase the effective output capacitor value. When using
bypass capacitors (for low noise operation), larger values
of output capacitors are needed. For 100pF of bypass ca
-
pacitance, 3.3µF of
output capacitance is recommended.
With a 330pF bypass capacitor or larger, a 4.7µF output
capacitor is recommended. The shaded region of Figure 2
defines the range over which the LT3032 is stable. The
minimum ESR needed is defined by the amount of bypass
capacitance
used, while the maximum ESR is 3Ω. These
requirements are applicable to both the positive and nega
-
tive linear regulator.
Give extra consideration to the use of ceramic capacitors.
Ceramic capacitors are manufactured with a variety of di
-
electrics, each with different behavior across temperature
and applied voltage. The most common dielectrics used
are specified with EIA temperature characteristic codes of
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are
good for providing high capacitances in a small package,
but they tend to have strong voltage and temperature
coefficients as shown in Figures 3 and 4. When used with
a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an
effective value as low as 1μF to 2μF for the DC bias voltage
applied and over the operating temperature range. The X5R
and X7R dielectrics result in more stable characteristics
and are more suitable for use as the output capacitor.
The X7R type has better stability across temperature,
while the X5R is less expensive and is available in higher
values. Care still must be exercised when using X5R and
X
7R capacitors. The X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
Z5U capacitors, but can still be significant enough to drop
capacitor values below appropriate levels. Capacitor DC
bias characteristics tend to improve as component case
size increases, but expected capacitance at operating
voltage should be verified in situ for a given application.
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress. In
a ceramic capacitor, the stress can be induced by vibra
-
tions in
the system or thermal transients. Tapping on the
ceramic
bypass capacitor with a pencil generated the noise
shown in Figure 5. Similar vibration induced behavior can
masquerade as increased output voltage noise.
OUTPUT CAPACITANCE (µF)
1
ESR (Ω)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
3 10
1762 F02
2 4 5
6
7 8
9
STABLE REGION
C
BYP
= 330pF
C
BYP
≥ 3300pF
C
BYP
= 100pF
C
BYP
= 0
Figure 2. Stability
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